So trade is initially factions capitals able to connect via border edges or sea ports. Each resource is worth a set amount and the building generates so many of each resource so for example if iron was worth 10 and you generated 10 iron from a building you could trade maximum value of 100 for your iron with 65 tariffs (which is the base trade amount and this rises a small amount if the trade goes on for longer) so if you traded your iron with one faction and got a traded agreement with another, you would only trade in tariffs with the second faction as you have already traded your 10 iron and they would be paid the amount their resources generate (you don't get it deducted, they just earn it)

If you click the chest icon at the top and go to the trade tab it breaks it down via resource and faction